Make Migrations::new const #993

Job	Run time
Clippy (all code)	12s
Code coverage	1m 30s
Check	13s
Clippy (main package)	39s
Documentation	13s
Check with minimal Rust version	45s
Mutation tests (0)	1m 16s
Rustfmt	5s
Mutation tests (1)	1m 5s
Test all features	34s
Test feature alpha-async-tokio-rusqlite	43s
Mutation tests (2)	1m 4s
Test examples	46s
Mutation tests (3)	1m 2s
Test feature from-directory	47s
Test no features	35s
	11m 29s

Provide feedback